1. Field of the Invention
The present invention relates to a process for the production of a molded glass article and, particularly, relates to a process for the production of a molded glass article from a glass material containing lead oxide.
2. Description of the Prior Art
Lots of attempts to produce an optical lens and the like with high accuracy without after-treatment such as polishing after press molding have been made in the recent years. One of the molding techniques is a process having the steps of: placing a glass material between upper and lower molds made of metals, cermets, ceramics, etc.; heating the glass material substantially to its glass softening point; and press-molding it. In general, the press molding is carried out in an atmosphere of a non-oxidizing gas such as a nitrogen gas. This reason is in that not only stainless steel and other materials which constitutes a press machine are prevented from oxidizing but also a mirror surface of the molds which contacts with the glass material is prevented from oxidizing in an oxidizing atmosphere. If the surface of the molds which have been polished like a mirror is oxidized, the surface is roughened so that the mirror property thereof is lost.
However, when a glass material containing about 20% by weight or more of lead oxide is press-molded in a non-oxidizing atmosphere of an oxygen concentration of 100 ppm or less, lead oxide (PbO) at the glass surface is reduced to other lead compounds or metal lead (Pb). As a result, the surface of the press-molded glass becomes cloudy. If the molds in such a condition are used repeatedly, a reaction product is generated by the interaction of the mold surface and the lead component of the glass surface on the mold surface so that the mirror property thereof is lost. No effective technique for dissolving the reaction product to return the mold surface to the original mirror surface has been found. Further, apart from this cloudiness, the following disadvantage is observed. That is, the surface of the glass material press-molded with such molds in which the mirror property has been lost is so rough that a mirror surface necessary in a press-molded article cannot be obtained.
As a technique for solving the problem observed when the lead oxide-containing glass is press-molded, Japanese Patent Unexamined Publication Sho-62-162629 discloses a process in which press-molding is carried out while successively increasing the oxygen concentration in the non-oxidizing atmosphere in each of a certain number of molding operations. However, when a lead oxide-containing glass is press-molded by the process described in the Japanese Patent Unexamined Publication Sho-62-162629, the cloudiness of the glass surface cannot be eliminated perfectly. Further, the process has a problem in that deposition of lead on the surfaces of the molds cannot be prevented perfectly.